The purpose of this project is to study the physical properties of a wide variety of biological macromolecules with the goal of correlating these properties to the structure and function of the macromolecules. The emphasis is on the thermodynamics of the interactions of these macromolecules and on their molecular size and shape. Analytical ultracentrifugation and mathematical modeling are the principal research techniques used. Studies on the association of fibrinogen with other proteins involved in blood clotting and fibrinolysis have been continued. Studies currently in progess in this area deal with the association of fibrinogen with plasma and platelet Factor XIII and with the association of plasminogen with the D and E fragments of fibrinogen. Studies on the association of the A and B chains of reduced ricin indicate that the formation of the AB cmplex is reversible and that the temperature dependence of this reaction indicates that it appears to be entropically driven, suggesting that there is a strong hydrophobic interaction between A and B chains even in the absence of the disulfide bond. Cell toxicity studies indicate that reduced ricin is more toxic than native ricin at high concentrations, indicating that the disulfide bond between the chains is not necessary for biologcial activity. The small and large forms of heparin sulfate proteoglycan isolated from EHS tumor basement membrane have been studied by electron microscopy and by analytical ultracentrifugation. These studies indicate that these proteoglycans show similarities in the number and size of the heparin sulfate side chains but differ in the length of the protein core.